Figure 1: Schematic illustration of the optical path of the TFI. (Click on the figure to enlarge.)

The TFI is a versatile, general-use camera that enables the JWST to image a 2.2' × 2.2' field of view through narrow-band (R∼100)filters. The central wavelengths of the filters can be selected by an observer over a wide range of near-infrared wavelengths. Although it is packaged with the Fine Guidance Sensor, the TFI is completely independent of the guide camera and plays no role in acquisition and tracking functions.

The novel capability of "tuning" filters to user-specified wavelengths is provided by a Fabry-Perot etalon and a suite of order-blocking filters. The optical coatings on the etalon plates permit the TFI to operate between 1.5 and 5 µm, with a small gap in coverage between 2.5 and 3.2 µm. Motions of the etalon are controlled by six piezoelectric actuators, which provide a high degree of stability and repeatability. The detector is a 2048 × 2048 HgCdTe array manufactured by Teledyne with 18 µm pixels and a composition tuned to provide a long-wavelength cutoff at approximately 5.3 microns. The TFI can be used for direct imaging or imaging with a choice of four coronagraphs or a non-redundant mask with full-frame or subarray readouts.

The TFI complements other JWST science instruments by providing images with the quality of NIRCam through filters with wavelength resolution and sensitivity comparable to the low-resolution mode of NIRSpec. It also provides the observatory with imaging capability over the wavelength gap caused by the dichroic beamsplitter in NIRCam. The ability of the TFI to scan in wavelength while keeping all other components of the instrument and observatory fixed provides vastly superior suppression of the "speckles" caused bymid-frequency distortions in the primary mirrors. Thus, the TFI is expected to be the instrument of choice for demanding coronagraphic observations in the near-infrared. The TFI is being developed for the JWST project by the Canadian Space Agency, which has selected COMDEV as the prime contractor.René Doyon (Université de Montréal) is the Principal Investigator and leader of the Science Team overseeing the construction of theTFI.